Ozone has a high oxidation potential and is used in a variety of fields for purposes such as pollutant degradation and sterilization. One of the devices that uses ozone is an exhaust purifying system for purifying exhaust gas of an internal combustion engine (hereinafter, referred to as an engine).
Systems using a selective reduction catalyst (urea SCR systems) are already in practical use as exhaust purifying systems. One such system is a proposed selective catalytic reduction system that adds ozone to a section upstream of a selective reduction catalyst with a urea fluid (e.g., refer to Patent Document 1). A portion of nitric monoxide (NO) contained in exhaust gas is converted to nitric dioxide (NO2) by adding ozone (O3) to the exhaust gas. This brings the ratio of NO to NO2 contained in the exhaust gas close to 1 to 1. When the ratio of NO to NO2 approaches 1 to 1, the reduction reaction of NO and NO2 to nitrogen (N2) accelerates.